Railcar couplers are used to connect railcars together. Typical couplers used throughout North America are the Type-EF and Type-F couplers, also referred to as SBE60, SBE69, and or E69 couplers, respectively. These couplers are normally produced through a sand casting process, which offers a low-cost, high-production method for forming complex shapes.
In sand casting, a mold is created using a sand and binder mixture (i.e., molding sand). The binder allows the sand to retain a shape. The most common sand/binder mixture used for casting couplers is green sand, which consists of silica sand, organic binders and water. Green sand is used primarily due to its lower cost.
The mold typically comprises a cope portion (i.e., top half) and a drag portion (i.e., bottom half), which are separated along a straight or offset parting line. To form the cope and drag portions, patterns that define cope and drag portions, respectively, of the coupler and a gating system are placed into separate flasks. Molding sand is then packed around the patterns, to define mold cavities for the coupler and gating system. Draft angles of 3 degrees or more are machined into the pattern to ensure the pattern releases from the mold.
In some instances, two or more couplers may be cast simultaneously by forming the mold around two or more patterns that define multiple couplers in a single flask. For example, two patterns may be arranged side-by-side within the flask. A common gating system may be formed along the longitudinal axis of each pattern and may be configured to feed molten metal to both the head and shank regions of the mold (i.e., those areas of the mold that will correspond to the head and shank sections.)
The patterns are then removed from the mold, and cores for defining various internal cavities of the coupler are placed into the mold. The mold is then closed and filled with hot liquid metal, which is poured into the mold via a down sprue.
After the metal has been poured into the mold, the casting cools and contracts as it approaches a solid state. Risers, which are reservoirs of molten material, are placed at those areas of the casting that exhibit the highest contraction. The risers feed those areas as the casting cools to help minimize the formation of voids, which would otherwise occur. The risers are formed in the cope portion of the mold and typically define openings, which may allows gasses to escape during pouring and cooling.
After solidification, the metal (i.e., raw casting) is removed by breaking away or burning off the rigging. The casting is then finished and cleaned via grinders, blasting, welding, heat treatment, or machining.
The casting techniques described above have several disadvantages. First, the binders used in the in the molding sand can have a significant effect on the final product, as they control the dimensional stability, surface finish, solidification, and casting detail achievable in each specific process. In particular, couplers cast in green sand have a relatively poor dimensional stability and surface finish. These couplers may also exhibit a higher rate of defects due to solidification issues.
As noted above, two or more couplers are sometimes formed in a single flask. However, the size of the flask has to be relatively large because of the way in which the gating system is arranged with respect to the coupler cavities. Other problems with these casting operations will become apparent upon reading the description below.